Fluoride fiber has rapidly developed into a very useful medium for mid-IR transmission, with great potential for long distance IR communications. Fluoride fibers are predicted to have intrinsic losses less than those of silica fibers, which might increase significantly the repeator spacing in long fiberoptic links, especially in undersea cables. Unfortunately, the chemical durability of fluoride glasses is low and, therefore, the mechanical failure of fluoride fibers over extended periods of time will be more likely than with silica fibers. Liquid and gaseous water react readily with the fiber surface, leading to the formation of surface flaws and stress corrosion.
Two solutions to the corrosion/strength problem have been proposed. The first one is to protect the fiber surface by an impervious hermetic coating, so that the water or moisture would be prevented from reaching the fiber surface. Various types of plastic, metal and ceramic hermetic coatings have been described in the prior art.
The second solution is to make the core glass of mixed fluorides and cladding of chalcogenide glass. Chalcogenide glasses have better chemical durability than fluoride glasses (A. Nakata, J. Lau and J. D. Mackenzie, Materials Science Forum, 6, 717 (1985). The principal disadvantage of chalcogenide glass is its high level of toxicity, which is very disturbing in the case of medical or surgical applications.
Another problem, common to both fluoride and chalcogenide glasses, is the necessity to protect them during the fiber draw by an inert atmosphere to avoid surface reaction with moisture or oxygen. In fluoride glasses reaction with moisture can cause surface defects and initiate crystallization (G. S. Sapsford, N.J. Pitt and J. D. Morris, paper presented at Soc. of Glass Technology Meeting, Peebles, Scotland, Jun. 12, 1986; H. Schneider, A. Schobeth and A. Staudt, paper presented at 4th Int. Symposium on Halide Glasses, Monterey, Calif., Jan. 28, 1987).
A common way of protecting fluoride fibers today is by applying a thin layer (5-30 microns) of a Teflon FEP coating. This layer enables the fiber to be handled but does not protect it for long periods of time against moisture, which can migrate through the porous coating.
The object of this invention is to protect the fluoride fiber by providing an additional overcladding of oxide glass or other hermetic coatings which are compatible with the fluoride glass.